Automatic transmissions have been utilized for various drive systems, such as in automobiles to allow for ease in driving. The demand for such an automatic transmission has been accordingly increasing from year to year. Conventional automatic transmissions utilize fluid torque converter systems. However, such a system is disadvantageous in many respects such as poor power transmission efficiency and complicated construction which leads to higher production costs. More recently, planetary gear type automatic transmissions have been utilized to eliminate some of the above-noted disadvantages.
In conventional belt type power transmission it has been desirable to obtain a broad speed range by use of a belt transmission system only without requiring the use of any subtransmission consisting of a planetary gear transmission or gear transmission unit. However, the necessary range of engine speed is generally as large as 1000 RPM to 6000 RPM, with a large load torque. In order for a belt to permit a large torque to be produced by an automobile engine, while the engine is running, it is necessary to increase the minimum effective pitch diameter of the V-belt to enable adequate performance.
When an automobile engine is running at high speed, the effective diameter of a speed change belt for the V-belt transmission system is maximized. Therefore, in proportion to the increase of speed range, the maximum effective pitch of the speed change pulley on the drive side must be increased by the amount of transmission ratio with respect to the allowable minimum effective pitch diameter of the belt when the torque on the belt is at its highest. As a result, the circumferential speed of the belts becomes extremely high when the engine speed is very high, and the centrifugal force caused by such a large circumferential speed prevents the belt from transmitting power effectively.
The present invention has been developed to overcome the above-described problems with prior power transmission systems.